1. Field of the Invention
The present invention relates to a semiconductor device including thin film transistors (TFTs), and a method for manufacturing the same. More particularly, the present invention relates to a semiconductor device including thin film transistors in which the active region is formed from a crystalline semiconductor film obtained by crystallizing an amorphous semiconductor film, and a method for manufacturing the same. The semiconductor device of the present invention, having thin film transistors formed on an insulative surface such as a glass substrate, can be used in various applications such as active matrix liquid crystal display devices, organic EL display devices, contact image sensors, and three-dimensional ICs, and other suitable apparatuses.
2. Description of the Related Art
In recent years, attempts have been made in the art to form high-performance semiconductor elements on an insulative substrate such as a glass substrate or an insulating film, aiming at realization of liquid crystal display devices and organic EL display devices having larger sizes and higher resolutions, contact image sensors operating at higher speeds with higher resolutions, three-dimensional ICs, etc. Particularly, a type of liquid crystal display device having the pixel section and the driving circuit on the same substrate is finding wide usage in various household appliances, in addition to a monitor of a personal computer (PC). For example, liquid crystal displays are used as television sets, replacing CRTs (Cathode-Ray Tubes), and front projectors are used for home entertainment applications such as for watching movies and for playing video games. Thus, the market for liquid crystal display devices has been growing at a remarkable rate. Moreover, system-on-panel devices have been developed actively, in which logic circuits such as a memory circuit and a clock generation circuit are formed on a glass substrate.
Displaying high-resolution images means an increase in the amount of data to be written to pixels, and the data needs to be written within a short time. Otherwise, it is not possible to display a moving picture that has a very large amount of data for high-definition display. Therefore, TFTs used in a driving circuit are required to operate at a high speed. In order to achieve high-speed operations, there is a demand for forming the TFTs using a crystalline semiconductor film having a desirable crystallinity, with which it is possible to obtain a high field-effect mobility.
The present inventors have developed a method for obtaining a desirable crystalline semiconductor film on a glass substrate. In this method, a metal element capable of promoting crystallization is added to an amorphous semiconductor film, which is then subjected to a heat treatment. With this method, a desirable semiconductor film having a uniform crystal orientation can be obtained through a heat treatment performed at a lower temperature and for a shorter time than other conventional methods.
However, when a silicon film crystallized with a catalyst element is used as it is as the semiconductor layer of a TFT, the TFT will have an abrupt increase in the off-state current. The catalyst element irregularly segregates in the semiconductor film, and the segregation is significant at crystal grain boundaries. It is believed that the segregation of the catalyst element creates leak paths for a current, resulting in the abrupt increase in the off-state current. Therefore, after the formation of the crystalline silicon film, it is necessary to reduce the catalyst element concentration in the semiconductor film by moving the catalyst element out of the semiconductor film. The step of removing the catalyst element will be hereinafter referred to as a “gettering process”.
Various types of gettering processes and methods have been proposed in the art.
For example, Japanese Laid-Open Patent Publication No. 8-213317 discloses a technique of forming an amorphized region in a silicon material that has been crystallized by using a catalyst element, and subjecting the silicon material to a heat treatment so that the catalyst element is moved (gettered) into lattice defects in the amorphized region. The publication discloses one method in which the amorphous region (gettering region) is formed outside the semiconductor element region, and another method in which the source/drain region of the TFT is used as the amorphous region (gettering region). When the source/drain region is used as the gettering region, the manufacturing process can be simplified. However, the method requires an additional step of activating the source/drain region with laser light, or the like, since an amorphous region as it is cannot function as a source/drain region.
Japanese Laid-Open Patent Publication No. 10-270363 discloses a technique of selectively introducing a group VB element such as phosphorus into a portion of a silicon material that has been crystallized by using a catalyst element, and subjecting the silicon material to a heat treatment so that the catalyst element is moved (gettered) into the region where the group VB element has been introduced (gettering region). As the catalyst element is gettered into the region where the group VB element has been introduced, there is created a region where the catalyst element concentration is lowered (hereinafter referred to also as “low-catalyst-concentration region”), and this region is used to form the active region of the semiconductor element (TFT).
Japanese Laid-Open Patent Publication No. 9-107100 discloses a method in which a silicide component of the catalyst element is selectively etched away by using hydrofluoric acid.
The conventional gettering processes, including those disclosed in the three publications mentioned above, have various problems such as the provision of additional steps for the gettering process, which complicates the manufacturing process and increases the load on the manufacturing apparatus, thereby increasing the cost.
However, the most serious problem which has not been recognized or addressed to date, is that the conventional methods do not provide a sufficient gettering effect, and are not capable of sufficiently lowering the amount of the catalyst element remaining in the channel region of the TFT. That is, the conventional gettering methods consistently left catalyst in the channel region and did not address the fact that there were several different types of catalyst (e.g. Si, Ni, NiSi, Ni2Si) remaining in the channel region that had to be removed to avoid serious problems with the resulting semiconductor films and elements (TFTs).
One of the simplest methods may be to etch away the catalyst element in the silicon film by using hydrofluoric acid, as disclosed in Japanese Laid-Open Patent Publication No. 9-107100. The present inventors have actually examined the amount of the catalyst element remaining after the gettering process of Japanese Laid-Open Patent Publication No. 9-107100, and discovered and confirmed through experiment that about one half of the catalyst element introduced into the silicon film remains unremoved. Even if the concentration of hydrofluoric acid is increased or the etching time is extended, the process reaches saturation when about one half of the catalyst element is removed, and the amount of the catalyst element cannot be reduced any further. Thus, the present inventors discovered and confirmed that this conventional gettering method is capable only of reducing the catalyst element concentration to about one half of that at the time of introduction of the catalyst element. As TFTs were produced with this method, about 10% to 20% (in terms of the number of products) of all the TFTs produced were defective with significant off-state leak current. This corresponds to 100,000 to 200,000 defective TFTs in an active matrix substrate (a form of semiconductor device) having 1,000,000 TFTs. Also about 10% to 20% of all the reference TFTs, produced with no gettering process at all, were defective with significant leak current, indicating that with the method disclosed in Japanese Laid-Open Patent Publication No. 9-107100 alone, the gettering process is not effective at all in improving the device characteristics.
In contrast, when a gettering region is formed by introducing a “gettering element” (the term “gettering element” as used herein refers to an element capable of attracting the catalyst element) such as an amorphous element or phosphorus so that the catalyst element in the silicon film is moved into the gettering region, as disclosed in Japanese Laid-Open Patent Publication No. 8-213317 or Japanese Laid-Open Patent Publication No. 10-270363, the present inventors discovered and confirmed that the amount of the catalyst element can be reduced by one order of magnitude or more. However, as TFTs were produced with the methods disclosed in these publications, there were defective TFTs with significant off-state leak current at a defect rate on the order of 0.1 to 1% for both of the methods, with a slight difference between their gettering effects due to the difference in the method of forming the gettering region. As about 10% to 20% of all the reference TFTs, produced with no gettering process at all, were defective with significant leak current, it can be seen that the methods of these publications clearly provide some gettering effect, improving the device characteristics. Nevertheless, there still are defective TFTs with significant leak current at a defect rate on the order of 1% even with these methods. This corresponds to some tens of thousands of defective TFTs in an active matrix substrate having 1,000,000 TFTs.
Thus, with the conventional gettering techniques, one needs to expect a TFT defect rate at least on the order of 0.1%, which is the lowest rate in the present inventors' experimental data. If an active matrix substrate for a liquid crystal or organic EL display device is produced with such a TFT defect rate, some pixel TFTs will have off-state leak current, resulting in bright spots (point defects), and the driver (driving circuit) section will have a line defect due to leak current in the sampling TFT section. As a result, the panel production yield will be decreased significantly.
An analysis has confirmed that a defective TFT with significant off-state leak current contains masses of a silicide of the catalyst element at the junction between the channel region and the drain region. Thus, the primary cause of the defect is the segregation of the catalyst element, and the secondary cause is the gettering of the catalyst element being insufficient. With the conventional techniques of the publications mentioned above, the catalyst element is not gettered sufficiently, a problem which was not recognized previously but was discovered and confirmed by the present inventors. Thus, even though these conventional gettering techniques are capable of producing some high-performance TFTs, with such high defect rates and poor reliabilities, they cannot be used for mass production.